A cement composition such as mortar and concrete etc. shows a decrease in consistency with passage of time after combination by the hydration of cement with water or others and thereby, the efficiency in the execution of works lowers. This phenomenon is generally called slump loss.
The slump loss in the cement composition causes, on ready mixed concrete, trouble such as limitation of time allowed for transportation, degeneration of quality and impairment of workability owing to a waiting time in a concrete-placing field, and a decrease in durability due to a cold joint and so on. Also, in a factory for producing concrete secondary products, when transportation of a cement composition by a pump pressure is temporarily stopped by taking a lunch time or due to any trouble and, some times later, the transportation by pressure is reopened, the slump loss may cause such an accident as a sudden increase in the transportation pressure or full closure of a pump and may cause such a problem as incomplete filling in a delaying case of moulding such as compaction for some reason. Therefore, in the factories for producing ready-mixed concrete and for producing concrete secondary products and in the other places, the slump loss in a cement composition is an important problem to be solved in order to control the quality of cement compositions and to improve the efficiency in the execution of work.
The following methods to prevent the slump loss have been proposed.
(1) A method in which an unit amount of water in concrete is increased.
(2) A method which comprises a post-addition of a cement additive.
(3) A method which comprises a repetitive addition of a cement additive.
(4) A method which comprises an addition of a retarder or a combined use with a cement additive.
(5) A method in which a cement additive of a granular type is used.
(6) A method in which a cement additive is contained in an organic hydrogel.
(7) A method which comprises an addition of a cement additive that undergoes slow-release of a dispersing effect for cement resulting from slow hydrolysis by an alkali.
Said method (1) comprises an increase of an unit amount of water which is estimated with the slump loss up to placing of a cement composition. Even if an improvement in the workability is attempted by this method, the method may be accompanied by such a disadvantage in quality as a loss of strength of a hardened product and crack occurrence resulting from shrinkage during drying, which causes a decrease in durability or such an economic disadvantage as an increase in an unit amount of cement which is required to obtain a defined strength.
Said method (2) is a temporary improvement for a flowability. However, the cement additive remains locally in the cement composition after an attempt to maintain the slump has been completed and, as a result, there will be brought a bad effect such as occurrence of local bleeding and thereby, a loss of strength.
Said method (3) comprises a readdition of an additive when the slump loss occurs. However and it is not a complete prevention for the slump loss. Also, there is a disadvantage in the workability and economy due to the repetitive addition.
Said method (4) seeks to maintain flowability by lengthening the time necessary for coagulation of a cement composition by using only a delaying agent such as a hydroxycarboxylate, a ligninsulfonate, a dextrin, and a humic acid etc. or by using those in combination with a cement additive and, therefore, in a case of an excess addition there is a danger of accidents such as a loss of strength and inferior setting.
The method (5) is the one in which a condensation product of naphthalenesulfonic acid with formalin is granulated and gradually dissolved in a cement composition to prevent the slump loss (Japanese Official Patent Provisional Publication, showa 54-139929) or the one in which a copolymer of an olefin with an ethylenic unsaturated dicarboxylic acid anhydride gradually undergoes hydrolysis in a cement composition to prevent the slump loss (Japanese Official Patent Provisional Publications, showa 60-16851 and heisei 1-122947). But in both these methods, the granular additive is dispersed in solution, the methods involve localization of the additive and a problem a storage stability. Furthermore, to prevent said localization of an additive and the problem of storage stability, there has been proposed a method in which a finely granulated cement additive is obtained by that a solution being dispersed with a copolymer of a vinylic compound with an ethylenic unsaturated dicarboxylic acid anhydride is stirred with a rigid body media (Japanese Official Patent Provisional Publication, showa 62-241855), but this method requires a device such as a sand mill for stirring, so that the production process becomes complex.
The method (6) is the one in which a cement-dispersing agent is contained in such an organic water-containing gel as polyacrylic acid etc. and it gradually releases in the cement paste (Japanese Official Patent Provisional Publication, showa 63-162562), but because it involves a gel which is not dissolved in water, there is a problem of storage stability such as separation and precipitation etc.
The method (7) comprises a method in which as a cement additive, a poly(meth)acrylic acid ester is used in combination with such a cement-dispersing agent as sodium ligninsulfonate, sodium melaminesulfonate, and sodium polyacrylate (Japanese Official Patent Provisional Publication, showa 60-161365), and a method in which dispersion of cement particles is in tended by using methyl poly(meth)acrylate or a copolymer of t-butyl maleate with isobutylene [A. Ohta, Y. Tanaka, and T. Uryu: Polymer Preprints, Japan Volume 38, No. 3 (1989)]. In the polymers which are used as cement additives of the forementioned kinds, the carboxyl groups directly binding to a carbon atom of the main chain are all esterified. Because of this, when the polymers are combined with a cement composition, they are hardly compatible with water and the cement composition becomes non-uniform and the setting inferiority etc. often takes place due to local existence of the additive.
In short, all the methods have their own defects and there has not been found any satisfactory cement additive.
Thus, the first object of the present invention is to provide a cement additive which, by being added to a cement composition, is able to improve said defects, enhance its flowability, and prevent the slump loss. The second object of the present invention is to provide a method for producing such a superior cement additive as forementioned and also, with which a molecular structural design is easy. The third object of the present invention is to provide a production method which can produce such a superior cement additive as forementioned by taking only a short process. Furthermore, the fourth object of the present invention is to provide a cement composition in which said superior cement additive is combined.